Centrifuge construction

ABSTRACT

A CENTRIFUGE HAS A CENTRIFUGE DRUM PROVIDED WITH A CIRCUMFERENTIAL WALL WHICH CARRIES ON ITS INTERIOR SIDE A COARSE-MESH SUPPORT SCREEN. A FINE-MESH COVER SCREEN OVERLIES THE SUPPORT SCREEN AT THE INTERIOR SIDE THEREOF AND COMPRISES A PLURALITY OF APERTURES WHICH ARE SO DISTRIBUTED   THAT ANY FOUR ADJACENT ONES OF THEM ARE LOCATED AT THE CORNERS OF A RIGHT-ANGULAR PATTERN.

K. PAUSE CENTRIFUGE CONSTRUCTION May 1, 1973 5 Sheets-Sheet 1 Filed June 14, 1971 FIG.1 V

INVENTOR Iwn PAW? Ill/7 May 1, 1973 K. PAUSE CENTRIFUGE CQNSTRUCTION Filed June 14; 1971 3 Sheets-Sheet 2 .May1,1973 K. PAUSE v 3,730,768

v CENTRIFUGE CONSTRUCTION Filed June 14, 1971 3 Sheets Sheet 3 INVENTOR.

ma BY kunr V and f/ fl United States Patent 3,730,768 CENTRIFUGE CONSTRUCTION Kurt Pause, Grevenbroich, Germany, assignor to Maschinenfabrik Buckau R. Wolf Aktiengesellschaft, Grevenbroich, Germany Filed June 14, 1971, Ser. No. 152,778 Claims priority, application Germany, June 13, 1970, P 20 29 261.9 Int. Cl. C13f 1/10 US. Cl. 127-19 7 Claims ABSTRACT OF THE DISCLOSURE A centrifuge has a centrifuge drum provided with a circumferential wall which carries on its interior side a coarse-mesh support screen. A fine-mesh cover screen overlies the support screen at the interior side thereof and comprises a plurality of apertures which are so distributed that any four adjacent ones of them are located at the corners of a right-angular pattern.

BACKGROUND OF THE INVENTION The present invention relates generally to a centrifuge construction, and more particularly to a centrifuge which operates discontinuously, particularly a sugar centrifuge.

Centrifuges of this type are already well known. Generally speaking, they have a circumferential wall on the centrifuge drum, and two opposite ends provided with covers. In the region of the opposite ends the circumferential wall has only a few outlet openings for the sugar syrup and the interior of the circumferential wall carries a coarse-mesh support screen which in turn carries a finemesh cover screen. Centrifuges of this type are used in a variety of applications, and as indicated above one of these is the production of sugar. When so used, the centrifuge serves for centrifuging of white sugar with increased capacity in the centrifuge at unchanged inertial moment.

In order to provide for particularly advantageous centrifuging, and in particular good separation, the wall of the drum has only relatively few apertures or perforations, with the few apertures for the outflow of syrup being located in the vicinity of the cover and bottom of the drum which are regions of low tensile stress. This is taught, for instance, in German allowed application 1,208,694 and as a result of this the circumferential wall of the drum can be given a wall thickness intermediate the zones of low-tensile stress-that is over approximately 90% of its axial lengthwhich is approximately 40% smaller than would otherwise be possible. This results in an increase of approximately 30% in the nominal tensile stress of the circumferential wall of the drum, and in a concomitant increase of the nominal tensile stress by the same amount in the cover screen.

Such cover screens are usually sheet metal members which are provided with fine perforations having a diameter on the order of substantially 0.5 millimeter, with the apertures being spaced at distances of 1 mm. This latter factor is particularly important for reasons of obtaining proper separation during centrifuging, meaning that the spacing of the apertures or perforations from one another should be as small as possible so that the free screen surface be as large as possible and the flow path be as small as possible.

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Cover screens of this type are inserted into the centrifuge drum and have at least one line of separation which extends in axial direction of the drum. This means that usch cover screens are not a circumferentially closed cylindrical element. However, during operation the load of the sugar or other crystalline or amorphous solid acting upon them under the centrifugal forces which develop, urges such cover screens into complete frictional engagement with the wall of the drum. In particular, in sugar centrifuge drums it is known that the mass pressure of the centrifuged crystals onto the cover screen amounts to between 17 and 27 kp./cm. with the result that the cover screen is firmly pressed into frictional engagement with the support screen and thus necessarily participates in the absolute expansion of the circumferential wall of the drum.

This is where some of the problems in the existing art arise. If the cover screen participates in the absolute expansion of the circumferential wall of the centrifuge drum, as outlined above, then a nominal tensile stress will occur in the cover screen which is directly proportional (relative to the tensile stress in the drum) to the moduli of elasticity of the drum material and the cover screen material. The periodic operation of such centrifuges, that is the intermittent or discontinuous operation thereof, subjects the cover screens as well as the drums to a repetitive tensile stress, and the elastic limit or even the break limit of the materials are only of secondary importance in this respect relative to the life expectancy of the components involved. What is of primary importance is the continuous strength, that is the time or endurance strength of the materials in question and this characteristic is known to be substantially lower than the elastic limit.

Another consideration is in that the heretofore customary perforations for cover screens has been found to have the most disadvantageous field characteristics in terms of the increased stress near the perforations, because the manner in which the perforations were arranged was selected purely with a consideration relative to free screen cross-section and small flow paths on the surface of the screen. These two considerations heretofore dictated that the perforations of the screen were arranged such that any four adjacent perforations defined a rhombohedral pattern namely the most disadvantageous in terms of the increased stress near the perforations of the screen. This is for instance disclosed in German Gebrauchsmuster 6,604,763. The substantially higher tangential tensile stresses in the screen of centrifuges having a drum wall with few perforations, made it important to consider the characteristics of the screen in relation to the increased stress near its perforations, and in particular of the cover screen.

It was found that these factors were diasdvantageous and required improvement, and the present invention provides such improvement.

SUMMARY OF THE INVENTION It is, accordingly, an object of the present invention to provide an improved centrifuge construction wherein the aforementioned disadvantages of the prior art are avoided.

More particularly it is an object of the present invention provide an improved centrifuge construction in which the pattern of arrangement of the perforations or apertures, in particular of the cover screen, is such as to avoid the disadvantages of the prior art.

Still more particularly it is an object of the present invention to provide such an improved centrifuge construction in which the characteristics of the cover screen with respect to the increased stress near the perforations are vastly better than what is known from the prior art.

A concomitant object of the invention is to provide such an improved centrifuge construction wherein the free screen cross-section is not significantly changed, so that the advantages obtained in the prior-art constructions with respect to separation efiiicency are retained while the advantages sought according to the present invention are simultaneously achieved.

In pursuance of the above objects and of others which will become apparent hereafter, one feature of the invention resides, in a centrifuge, particularly in a discontinuously operating sugar centrifuge, in the combination of a drum having a circumferential wall, a coarse-mesh support screen carried by the circumferential wall at the interior of the drum, and a fine-mesh cover screen overlying the support screen. According to the invention the cover screen comprises a plurality of apertures which are so distributed that any four adjacent ones of these apertures are located at the corners of and define with one another a right-angular pattern.

With the screen constructed in accordance with the present invention, the screen can withstand at least one campaign without damage due to the improved characteristics of the screen with respect to the increased stress near the apertures.

Tests which have been conducted on screens of the type in question, wherein any four adjacent apertures define a rhombohedral pattern as known from the prior art, determined that a hole factor on of 3.6 and a field quality factor of 1.5 existed. The web weakening, that is the weakening between the adjacent apertures, was 0.53. However, in the area of repetitive tensile stresses the web weakening may amount only to 0.2 if the perforated material is to have lasting strength. For time-dependent strength the value may be 0.4. It was found that in the existing screens wherein any four adjacent apertures or perforations define a rhombohedral pattern that the effect of the increased stress near the apertures or perforations amounted to a factor for increasing the nominal tensile stress to the maximum increased tensile stress near the apertures or perforations of =3.3. Thus, when in the heavily perforated but thick-walle drum a tangential tensile stress of 13 kp./mm. occurred, this meant a tangential tensile stress or nominal tensile stress in the screen of 6.2 kp./mm. and with the increased stress near the apertures or perforations amounting to a factor of 3.3, a maximum increased tensile stress at the screen perforations of 20.5 kp./mm. In the existing drums these relationships could be withstood safely over a prolonged period of time.

However, in the newer drums which are non-perforated except in the region of their top and bottom axial ends, a nominal tensile stress of 17 kp./mm. is found, and as a result the screens undergo a tensile stress of 8.1 kp./mm. and when the arrangement of the perforations in the screens is such that any four adjacent perforations define a rhombohedral pattern, the screens are subjected to a maximum increased tensile stress near the perforations of 26.7 kp./mm.

What this means is that this last-mentioned factor, namely the maximum increased tensile stress near the perforations of 26.7 kp./mm. which acts upon the screens, is already in excess of the elastic limit, so that permanent strength or even strength which is time-dependent no longer exists. As a result the prior-art cover screens when used in centrifuge drums which are non-perforated in the circumferential wall except in the region of the upper and lower axial ends, do not stand up for any prolonged period of time and have been found to tear after only a few days of use.

The present invention as briefly outlined above, avoids these problems and provides a cover screen which has a significantly prolonged lifetime and at the same time provides the same separating efficiency as that known from the prior art.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary developed view, on an enlarged scale, of a portion of a cover screen according to the present invention;

FIG. 2 is a table illustrating the characteristic data of known cover screens wherein any four adjacent apertures define a rhombohedral pattern, and of the screen according to the present invention; and

FIG. 3 is a somewhat simplified view of a centrifuge construction With a cover screen according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Discussing now the drawing in detail, and firstly FIGS. 1 and 3, it will be seen that the centrifuge drum which is illustrated and identified with the reference designation D, carries on its interior a coarse-mesh support screen S, which in turn supports on its interior a cover screen 1. This cover screen is provided with apertures or perforations 2 which, according to the present invention, are arranged in such a pattern that they are located at the corners of a right-angular field, that is any four adjacent ones of the apertures or perforations 2 are located at the corners of such a field. In the illustrated embodiment the arrangement is such that the distances t between adjacent apertures 2 in circumferential direction of the centrifuge drum d are smaller than the distances t between apertures 2 in axial direction of the drum D. Reference character d identifies the diameter of the apertures 2 and reference character A the maximum flow path of the separated-out liquid which is removed during centrifuging.

For purposes of explanation, an exemplary embodiment will be assumed. It will be assumed that the diameter d of the apertures or perforations is 0.55 mm., that the distances or spacing between apertures in circumferential direction of the drum i is 0.9 mm., and that the same spacing but in axial direction of the drum t,, is 1.5 mm.

With an embodiment wherein the above values are incorporated, the requirement is met that the values t =O.9 mm. be smaller than t,,=1.5 mm, and that the relationship 5 be smaller than one.

The relationship of t /t is designated with T and in the assumed example is The product of 6 ,-1- (0.61'0.6=0.366) is smaller than 0.4, and this product is also designated as Web weakening, mentioned earlier in this disclosure. With the aforementioned values, the maximum flow paths amount to 0.6

A screen having the aforementioned values has a free screen cross-section of 17.6%. However, because of the advantageous field arrangement the relationships or characteristics with respect to the increased stress near the apertures are substantially better. The aperture factor 0: is 1.76, the field quality 1.3 and the total effect of the increased stress near the apertures amounts to a factor of only 1.63 at a web weakening of 0.366.

A glance at FIG. 2 indicates that the corresponding values in a screen having an arrangement wherein any four adjacent apertures define a rhombohedral pattern, and assuming the same diameter of the apertures and the same flow paths, will have a maximum increased tensile stress near the apertures of 20.5 kp./rnm.

If one assumes this increased tensile stress near the apertures as a basic factor, and utilizes a screen according to the present invention, the nominal tensile stress of the drum can be increased to 26.7 kp./rnm. and the nominal tensile stress of the cover screen to 17 kp./mm. without destroying the cover screen in continuous operation. Because the nominal tensile stress at the earlier mentioned centrifuge drums amounts only to 17 kp./mm. the nominal tensile stress in the cover screen (if the latter is of the type according to the present invention) increases only to 8.1 kp./mm. and as a result of the lower maximum effect of the increased stress near the apertures which amounts to a factor of only 1.63 the maximum increased tensile stress near the apertures in the screen will then only be 13.2 kp./mm.

It will be seen from this juxtaposition of a screen according to the present invention with a screen according to the prior art that screens utilizing the present invention can be subjected to substantially higher stresses than those known from the prior art, with the result that they can be used even in centrifuge drums which are made of synthetic plastic material with a substantially lower modulus of elasticity. In fact, contrary to what is known from the art, screens according to the present invention could be made even of synthetic plastic material themselves, despite the fact that such materials are capable of withstanding only a substantially lower nominal tensile stress than screens made of metallic material. Also, the screens according to the present invention can be utilized in continuously operating centrifuges, which is not possible with those according to the prior art.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the types described above.

While the invention has been illustrated and described as embodied in a centrifuge construction, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. In a centrifuge, particularly in a discontinuously operating sugar centrifuge, in combination, a drum having two opposite ends and a circumferential wall which includes in the region of the respective opposite ends two end portions at least one of which is perforate, and an imperforate intermediate portion located between said end portions; a coarse-mesh support screen carried by said circumferential wall at the interior of said drum; and a finemesh cover screen overlying said support screen and comprising a plurality of apertures so distributed that any four adjacent ones of said apertures are located at the corners of and define with one another a right-angular pattern.

2. In a centrifuge as defined in. claim 1, wherein the maximum flow path distances of liquid within the respective pattern are smaller than 1 mm.

3. In a centrifuge as defined in claim 1, said drum having an axial and a circumferential direction, and whereinthe distance between adjacent ones of said apertures in said circumferential direction is at most equal to the distance between adjacent apertures in said axial direction.

4. In a centrifuge as defined in claim 3, wherein said distance in said circumferential direction is smaller than said distance in said axial direction.

5. In a centrifuge as defined in claim 3, said apertures having a diameter; and wherein the product obtained from the relationship of said distance in circumferential direction to said distance in axial direction on the one hand, and the relationship of said diameter to said spacing in circumferential direction on the other hand, is smaller than 0.4.

6. In a centrifuge as defined in claim 3, said apertures having a diameter; and wherein the relationship of said diameter to said distance in said circumferential direction is smaller than 1.

7. In a centrifuge as defined in claim 6, wherein said relationship approaches 1.

References Cited UNITED STATES PATENTS 2,096,594 10/1937 Sanchez y Cil.

1,946,500 2/1934 Roberts 210-380 2,028,168 1/1936 Roberts 210--3S0 FOREIGN PATENTS 1,208,694 1/1966 Germany.

MORRIS O. WOLK, Primary Examiner S. MARANTZ, Assistant Examiner US. Cl; X.R. 

